1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system, such as a copying machine, a printer, and a facsimile machine.
2. Description of the Related Art
Conventionally, in an image forming apparatus, toner left on a photoreceptor after transfer step is removed from the surface of the photoreceptor (image bearing member) by a cleaner, and remained in the cleaner to become a waste toner. However, from the point of environmental preservation and effective use of resources, it is desirable to prevent such a waste toner from being generated.
Therefore, in recent years, a “cleanerless system” image forming apparatus has been put to practical use, which is so designed that the cleaner is eliminated, then transfer residual toner on the photoreceptor is removed from the photoreceptor by “cleaning simultaneous with developing” by means of a developing device, and the removed toner is recovered by the developing device and reused.
In the cleaning simultaneous with developing, in the following developing step of the transfer residual toner on the photoreceptor after transfer, namely subsequently the photoreceptor is charged and exposed to form an electrostatic latent image. The transfer residual toner that exists on a part of the photoreceptor (non-image portion) which should not be developed by toner is recovered into the developing device by a fog removing bias in the process of the developing step of the electrostatic latent image. The fog removing bias means a fog removing potential difference Vback, being a potential difference between a DC voltage applied to the developing device and the surface of the photoreceptor.
According to this system, the transfer residual toner is recovered by the developing device and reused for the development of the electrostatic latent image in the following step. Therefore, the waste toner can be eliminated, with less trouble in maintenance. In addition, the apparatus is formed in the cleanerless system, and this contributes to reducing a size of the image forming apparatus.
In the above-described conventional image forming apparatus of cleaning simultaneous with developing, there is the one using a contact electrostatic charger, with a charger for charging the photoreceptor abut on the photoreceptor to apply charging process to the surface of the photoreceptor. In this case, when the transfer residual toner on the photoreceptor is passed through a charging portion, being a contact nip portion between the photoreceptor and the contact electrostatic charger, the toner is stuck to the contact electrostatic charger, with particularly a charge polarity in the transfer residual toner charged in an opposite-polarity opposite to a normal polarity. Thus, beyond an allowable range of the contact electrostatic charger is stained by toner, thereby causing a charging defect.
Namely, the toner, with the charge polarity charged reverse opposite to a normal polarity, is mixed in the toner as a developer, although little in amount. In addition, even in a case of the toner with charge polarity charged in the normal polarity, under an influence of a transfer bias and a separating discharge, the charge polarity is sometimes reversed or deelectrified, resulting in small charge amount.
Accordingly, the toner with the charge polarity charged in the normal polarity, reversal toner charged in the opposite-polarity opposite to the normal polarity, and the toner with little charge amount, are mixed in the transfer residual toner. The reversal toner and the toner with little charge amount are easily stuck to the contact electrostatic charger, when passing through the charging portion, being the contact nip portion between the photoreceptor and the contact electrostatic charger.
In addition, in order to remove/recover the transfer residual toner on the photoreceptor by the cleaning simultaneous with developing of the developing device, the charge polarity of the transfer residual toner on the photoreceptor must be the normal polarity, the toner in this case being passed through the charging portion and conveyed to the developing portion. Also, the charge amount in this case must be the charge amount of the toner whereby the electrostatic latent image of the photoreceptor can be developed by the developing device. It is impossible to remove/recover the reversal toner and the toner with inappropriate charge amount into the developing device from the surface of the photoreceptor, resulting in a cause of an image defect.
As described above, and as described in Japanese Patent Application Laid-Open No. 2002-99176, the transfer residual toner is charged in the normal polarity by a toner charging member, to arrange charging polarities in the normal polarities, and also the charge amount is made uniform. Thus, the toner can be prevented from sticking to the contact electrostatic charger.
However, the transfer residual toner, to which the charge is imparted by the toner charging member to prevent the toner from sticking to the contact electrostatic charger, has a greater charge amount than that of the toner capable of developing the electrostatic latent image of the photoreceptor. Therefore, this toner is hardly removed/recovered by the cleaning simultaneous with developing in the developing device. In this case, the toner remained in the photoreceptor is overlapped on the next image (recovery ghost), thus causing the image defect.
In order to prevent such an image defect, another toner charging member is installed on the upper stream side of the toner charging member along a rotation direction of the photoreceptor, thereby applying charging processing of charging in the opposite-polarity opposite to the normal polarity, to the transfer residual toner, and the charge amount of the transfer residual toner is controlled. Thus, the charge amount of the transfer residual toner can be controlled, then recovery of the toner by the developing device can be efficiently performed, and image staining due to recovery failure can be reduced.
However, when a residual toner amount after transfer is increased, toner amount accumulated in the toner charging member is increased, and the control of the charge amount of the transfer residual toner becomes unstable. Therefore, as described in Japanese Patent Application Laid-Open No. 2003-316202, a method of ejecting a stained transfer residual toner from the toner charging member to the photoreceptor by utilizing the potential difference between the photoreceptor and the toner charging member, is known.
Meanwhile, in the transfer step, a transfer condition in the transfer step is set so that a highest possible amount of toner on the photoreceptor is transferred, namely so that transfer efficiency is increased. A conventional transfer condition is determined corresponding to a resistance fluctuation of a transfer member. Meanwhile, the transfer efficiency is fluctuated according to a state of the toner developed on the photoreceptor. For example, when charge amount of the developed toner is small, the toner amount on the photoreceptor is hardly transferred, thus deteriorating the transfer efficiency. Reversely, when the charge amount of the developed toner is excessively great, the transfer efficiency is similarly deteriorated.
Therefore, in a structure of setting the transfer condition from the fluctuation of the transfer member, without taking into consideration the state of the developed toner, fluctuation of the transfer efficiency occurs. When the residual toner amount after transfer is increased, the toner amount accumulated in the toner charging member is increased, and the control of the charge amount by the toner charging member easily becomes unstable. With a high ejecting frequency, the toner amount accumulated in the toner charging member can be decreased. However, further efficient technique is desired.